The present invention has, as its object, self-supporting, highly porous molded parts, made by the methods of powder metallurgy, consisting of metallic or nonmetallic solids, processes for their production, and the use of the molded parts according to the invention.
Porous, molded ceramic parts are known in and of themselves. They are produced, for example, by the use of organic foam elements, which are saturated with a ceramic slip. After the organic components have been dried and burned off, a porous molded ceramic body remains behind as a negative of the foam element (so-called xe2x80x9clost formxe2x80x9d process; DE-OS 23 01 662). Another possibility consists in the direct foaming of a ceramic slip in a mold by the use blowing agents, followed by drying. It is also possible to use organic fillers as pore-forming agents in ceramic compositions for the production of porous molded bodies. The porous foam bodies known according to the state of the art suffer from the following disadvantages:
there is a limit on the achievable pore fraction;
a closed porosity is formed;
the pore distribution is inhomogeneous on the microscopic scale;
a large amount of organic filler is required;
the production process is very time-consuming; and
the processes are limited to fine powders.
A process for the production of fine-pored solid bodies with a high pore volume is described in DE-OS 41 02 430. In this process, a coarsely dispersed, sedimentable mixture of a liquid phase and solid particles is allowed to separate by sedimentation, and the sediment is solidified in the presence of the liquid phase by a chemical reaction between the sediment particles to form a porous body, which has sufficient dimensional stability for a heat treatment. These solid bodies are then sintered at a temperature of preferably more than 1,000xc2x0 C. The disadvantage of this process is that the liquid phase and the sediment particles must be chosen to cooperate with each other in such a way that a chemical reaction will occur between the sediment particles. The sediment particles are bonded to each other directly by the chemical reaction, i.e., without a binder. As a result, monophase, fine-pored solid bodies are obtained.
Solid, porous ceramic bodies have many uses. They are suitable, for example, for use as light-weight construction blocks; as support material for other liquid or solid phases; and as insulation, construction, filler, or filter material. A special area of application for porous molded ceramic bodies has opened up only recently. To reduce the total weight of motor vehicles, increasing use is being made of light-weight construction materials in passenger cars. To ensure the necessary strength values, porous ceramic molded bodies are infiltrated with light metal alloys.
The task of the present invention is to provide porous molded ceramic bodies which can be produced by the methods of powder metallurgy and which do not suffer from the disadvantages of the known porous molded bodies according to the state of the art.